Hybrid Incremental Sheet Forming Methods for Enhanced Process Performance and Material Properties

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Demands in the aerospace, automotive and biomedical sectors for low-volume sheet metal parts made from materials with high specific strength are growing due to the needs imposed by rapid product development cycles, and personalized products to mention a few. However, high strength-to-weight ratio materials, such as Ti6Al4V, are usually difficult to form at room temperature necessitating the use of complex and costly hot forming processes. Alternatively, processes with combined actions of local deformation and heating, such as Electrically-assisted Incremental Sheet Forming (EA-ISF), are attractive when compared to conventional methods due to their increased process flexibility, decreased equipment capacity requirements, and shortened production cycles. In EA-ISF, the absence of a geometry-specific punch and die makes this process ideal for industries with small batch production runs. A few technical issues, however, hinder the wide commercialization of EA-ISF. Major roadblocks include the limited geometric accuracy of the final parts, unstable electric circuits during processing and the lack of in-depth knowledge on the material’s responses during processing and on the achieved product properties such as formability, hardness, surface topography and fatigue. Additionally, the dominant mechanisms behind electrically-assisted manufacturing processes remain a debatable research topic. ', 'To address the aforementioned issues, this work aims to: 1) improve the geometric accuracy of the final parts along with process efficiency, 2) realize the capability of deforming materials that are hard-to-form at room temperature, 3) unveil the dominant mechanisms behind the coupled electro-thermo-mechanical loading, and 4) enhance the performance of materials processed by EA-ISF.', 'Specifically, this thesis develops a novel hybrid ISF process that synergizes the desirable features of two different ISF processing modes. This strategy, in which the material is pre-strained followed by a low-force fine-tuning process, ensures the robustness, doubles forming efficiency and secures contact between the tools and the sheet metal. As such it was used as the basis for the development of two alternative variants of hybrid EA-ISF processes. Using this method, the forming depth has been increased by 115% compared to conventional EA-ISF. A practical post-forming annealing process that significantly increases geometric accuracy (up to 95%) is developed thereafter. This thesis discovers that apart from sparks, large forming forces limit the capability of deforming hard-to-form material in EA-ISF as well. ', 'To identify the dominant deformation mechanisms under electro-thermo-mechanical loading, a novel in situ characterization method for the material’s microstructure responses under macroscopic EA tensile loading is established. Combining both the material’s macroscopic and microstructural responses, this thesis refutes the existence of electrical-specific athermal effects in Ti6Al4V, regardless of the different patterns of electricity applied. Finally, an in-depth investigation of the resulting microstructures and material properties, obtained in ISF processes, is carried out. This thesis reveals the key factors that affect the material’s formability, hardness, surface quality, and fatigue life.', 'The ability to easily and rapidly manufacture sheet metal parts with the desired geometric accuracy and material properties, will unlock an entirely new design domain in product development that will allow for the continuing improvements in the processing and use of strong, lightweight parts in manufacturing industries. The methods and results generated in this work will contribute to the understanding and development of other manufacturing processes that are flexible or hybrid in nature. These could include processing ideas such as electrically-assisted rolling, electrically-assisted tube forming, on-site annealing and electrically-assisted vibrational surface treatment.

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  • 10/28/2019
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